The present invention relates to a method for controlling a low-pressure bypass system for a steam turbine installation having a reheater, in particular for run-up from the cold condition, for idling and low-load operation and after load rejection procedures.
Steam turbine installations with reheaters are steam turbines in which the steam emerging from turbines of certain pressure stages is heated again in a reheater before its introduction into a turbine with lower inlet pressure and, after passing the lower pressure units, is supplied to a condenser.
In order to start the block, it has been found advantageous, for various reasons and as a function of the mode of operation, to initially load the boiler to a minimum output by means of a steam bypass system, and then to begin the turbine starting procedure. Thus, in the case of a cascading bypass or wet reheater bypass system, the live steam is guided by means of a high-pressure bypass valve with downstream water injection to the xe2x80x9ccoldxe2x80x9d reheater rail and, after superheating in the reheater, is led from the xe2x80x9chotxe2x80x9d reheater rail via a low-pressure bypass valve into the condenser. This bypass system is also employed if, after a load rejection (for example when the mains circuit-breaker is opened) or in the case of a rapid shut-down of the turbine, the steam generated cannot be completely accepted by the turbine or cannot be accepted at all. Whereas the high-pressure bypass system depends essentially on the requirements of the boiler, the design and control of the low-pressure bypass system is determined by the steam turbine. The core feature in controlling a low-pressure bypass control system is the pressure control of the hot reheater main. As a rule, the required value control for the low-pressure bypass system consists of a minimum pressure and a floating pressure characteristic curve, which are triggered by a choice of maximum.
In the past, there were various concepts for forming the floating pressure characteristic curve for the required value control for the reheater pressure:
Derivation from the output.
Derivation from the pressure before the blading of the high-pressure turbine.
Derivation from the reheater steam flow, which can be calculated or measured.
A common feature of all these concepts is the fact that a certain required value characteristic curve is formed by means of suitable measures, which required value characteristic curve is located, during turbine operation, above the xe2x80x9cnaturalxe2x80x9d floating pressure characteristic curve, which is determined by the swallowing capacity of the medium-pressure turbine, so that the low-pressure bypass station remains closed.
The invention is therefore based on the object of making available a method for controlling a steam turbine installation having a reheater arranged between high-pressure turbine and medium-pressure turbine or low-pressure turbine, the steam turbine installation also comprising a low-pressure bypass with a low-pressure bypass valve, which low-pressure bypass leads from the reheater outlet into a condenser. The method is intended to permit the determination of the control of the reheater pressure value required for the low-pressure bypass station in such a way that, on the one hand, the low-pressure bypass station remains closed in turbine operation but, on the other hand, account is taken of the requirements arising from the starting and low-load operation of the high-pressure steam turbine so that, for example, windmilling of the high-pressure turbine is prevented or limited.
The present invention achieves this object in that characteristic curves for the required value of the reheater pressure are used for controlling the low-pressure bypass valve during run-up, during (partial) load rejection procedures or during idling, which characteristic curves depend on the load applied to the installation, and/or on the pressure before the high-pressure turbine blading and/or on the reheater steam flow, and also on the high-pressure turbine exhaust steam temperature, and/or on the temperature and/or on the pressure of the live steam introduced into the high-pressure turbine, and/or on the reheater pressure.
The core of the invention therefore consists in the provision not only of one single characteristic curve for the required reheater pressure value of the low-pressure bypass device as a function of one of the parameters known for this from the prior art, namely the load applied to the installation and/or the pressure before the high-pressure turbine blading and/or the reheater steam flow but, rather, of a characteristic curve field which, on the basis of one or a plurality of the previously mentioned parameters according to the prior art, additionally determines the required reheater pressure value as a function of the high-pressure exhaust steam temperature from the high-pressure turbine and/or of the temperature and/or of the pressure of the live steam introduced into the high-pressure turbine, and/or of the reheater pressure in the cold reheater rail.
In this arrangement, the swallowing capacity of the low-pressure bypass station is preferably designed in such a way that the associated floating pressure characteristic curve is located below the natural floating pressure characteristic curve, which is determined by the swallowing capacity of the medium-pressure turbine, when the low-pressure bypass valve is fully open.
According to a first preferred embodiment of the method, the characteristic curves are located, as a function of the load applied to the installation and/or of the pressure before the high-pressure turbine blading and/or of the reheater steam flow, between a maximum characteristic curve, determined by the required pressure value during turbine operation, and a minimum characteristic curve, determined by the minimum permissible pressure at full reheater flow, for different values of high-pressure turbine exhaust steam temperature and/or of the temperature and/or of the pressure of the live steam introduced into the high-pressure turbine and/or of the reheater pressure. The maximum characteristic curve determined by the required pressure value during turbine operation is typically located in this arrangement approximately 5 percent above the natural floating pressure characteristic curve, which is determined by the swallowing capacity of the medium-pressure turbine. In addition, the characteristic curves have, in the usual manner, a maximum value which is provided by the opening pressure of the safety valves.
According to a further preferred embodiment of the method, the characteristic curves are essentially linear with respect to the load applied to the installation, and/or with respect to the pressure before the high-pressure turbine blading, and/or with respect to the reheater steam flow. In a further preferred embodiment, the characteristic curves are also essentially linear with respect to the high-pressure turbine exhaust steam temperature, and/or with respect to the temperature and/or with respect to the pressure of the live steam introduced into the high-pressure turbine, and/or with respect to the reheater pressure. In addition, the characteristic curves preferably extend through a common origin, i.e. rotations about this common origin result for different values of high-pressure turbine exhaust steam temperature, and/or temperature and/or pressure of the live steam introduced into the high-pressure turbine, and/or reheater pressure.
According to another preferred embodiment of the method, the pressure in the cold reheater main before reheater inlet is used as a further control parameter for determining the characteristic curves.
In a particularly preferred method, the required value for the high-pressure exhaust steam temperature is determined by the live steam temperature, the high-pressure exhaust steam temperature being controlled to a value which is located around a constant value of preferably, in particular, in the region of 100 degrees Celsius below the live steam temperature. In this arrangement, the required value of the high-pressure exhaust steam temperature is always kept, however, within a range between a maximum value and a minimum value, the minimum value being typically located in the region of 320 degrees Celsius and the maximum value typically in the range from 420 to 450 degrees Celsius. Specifically, this means that in the case of live steam temperatures above 520 degrees Celsius, for example, the high-pressure exhaust steam temperature is always controlled so that it is limited to a value of 420 degrees Celsius as a maximum, and that in the case of live steam temperatures below 420 degrees Celsius, it is always controlled to a temperature limited to a value of 320 degrees Celsius as a minimum.
According to a further preferred embodiment of the above method, the control of the low-pressure bypass valve is carried out, with simultaneous trimming of the high-pressure turbine and the medium-pressure turbine or low-pressure turbine, by means of an inlet valve arranged in the live steam main before the high-pressure turbine and an intercept valve arranged before the medium-pressure turbine, the trimming condition of the high-pressure and medium-pressure/low-pressure turbine being, in particular, likewise preferably employed for determining the relevant characteristic curves. The trimming alone, such as is described in U.S. Pat. No. 4,132,076, for example, can lead to complete closing of the intercept valve under certain conditions, the result of which is that steam no longer flows through the medium-pressure turbine and low-pressure turbine, and the minimum cooling steam flow is no longer ensured for medium-pressure/low-pressure. The combination with the control by means of the low-pressure bypass valve can prevent this because, by this means, the high-pressure exhaust steam temperature can be reduced without the intercept valve having to be completely closed.
Further preferred embodiments of the method according to the invention are given in the sub-claims.
In addition, the present invention relates to an appliance for carrying out the method mentioned above. The appliance is, in particular, characterized in that a unit is arranged to calculate the required reheater value, which unit calculates, from the process values of the steam turbine installation influencing the characteristic curves, the characteristic curves corresponding to these values and controls, by means of a bypass control unit, the low-pressure bypass valve as a function of the process. In addition, a high-pressure/medium-pressure trim unit is preferably arranged for trimming the high-pressure turbine and medium-pressure turbine, which trim unit can supply the trimming condition to the unit for calculating the required reheater value.
Further preferred embodiments of the appliance according to the invention are given in the sub-claims.